The present invention relates to a spread spectrum communication system and handover method therein and, more particularly, to a spread spectrum communication system and handover method therein, which can quickly demodulate signals in a mobile station or handover destination site without performing a path search in a wide range.
With recent advances in mobile communication systems, so-called multiple access techniques of connecting a plurality of mobile stations to one base station to allow users as many as possible to simultaneously communicate within a limited frequency band have been developed.
As one of such multiple access schemes, a circuit switching scheme of holding the connection state set between two stations that communicate with each other up to the end of communication is popular.
This circuit switching scheme includes FDMA (Frequency Division Multiple Access) as a multiple access scheme using frequency division, which divides a frequency and allows users to respectively use the divided frequencies.
TDMA (Time Division Multiple Access) or the like is also available, in which a plurality of channels ensured by dividing time into small units are allocated to users.
Demands, however, have arisen for further improvements in current communication techniques, and more specifically, improvements in frequency use efficiency, security, interference resistance, and the like.
As a multiple access scheme capable of improving security, interference resistance, and the like by coding signals in channels as compared with FDMA, TDMA, and the like, a multiple access scheme called CDMA (Code Division Multiple Access) or SSMA (Spread Spectrum Multiple Access) is available.
In the past, the frequency use efficiency of spread spectrum multiple access was recognized to be lower than that of FDMA, TDMA, and the like. With advances in technology, we do not have such recognition any more and consider this technique as a technique capable of improving the frequency use efficiency.
In CDMA or SSMA, owing to coding processing, the bandwidth (frequency spectrum) of each narrow-band channel multiplies 10- to 100-fold.
With this technique, a plurality of signals having a spread frequency spectrum share the same band, but the respective channels are identified by differences in code pattern.
By executing operation (despreading) reverse to the coding processing that has been executed, the band of each channel is restored to the initial narrow band.
In this case, since the frequency spectra of signals from other stations are kept spread, a small component is left as noise in each signal restored to the narrow band.
Since this code pattern is complicated, it is difficult to decode the code. This also provides a kind of encryption communication.
In addition, a mobile communication system based on the spread spectrum scheme spreads/modulates an information signal with pseudo-orthogonal code (e.g., a PN code).
Reception waves can therefore be received with a precision corresponding to the spread code rate, and reflected waves with different propagation delays can be selectively received.
In addition, if a plurality of reception signals (multipath) with different propagation delays are selectively received and synthesized (RAKE synthesis), the resistance to multipath fading can be increased as compared with a conventional scheme of receiving only principal waves (e.g., the TDMA scheme).
In CDMA or SSMA, however, each path must be acquired with a precision corresponding to the spread code rate, and hence a high-precision path acquisition function is required.
When the phase timing of a spread code for a transmission signal coincides with that of a spread code for a receiver, a reception power peak appears.
Whether a path is acquired can be checked by detecting such a reception power peak.
For example, a phase timing at which a peak appears is detected while the phase timing of a spread code for the receiver is shifted in units of chips. This is called a path search function.
When a mobile station and base station start communication with each other, since the location of the mobile station in a site is unknown, a path search is made in a reception timing window range in which there is a possibility that a reception wave propagates. This operation is called path acquisition.
Once a path is acquired, the reception timing window is narrowed, and path detection is performed in this range. This operation is called path tracking.
Since this reception timing varies depending on the radio wave propagation time between a base station and mobile station, the reception timing changes when the mobile station moves to an adjacent site in handover.
For this reason, path acquisition must be performed again in handover between sites.
In handover, a short break occurs during execution of path acquisition because no signal can be received from the mobile station.
In the worst case, no path can be acquired, and handover may fail.
In addition, during soft handover, selective diversity cannot be performed during the execution of path acquisition.
An example of the arrangement of a conventional spread spectrum communication system will be described below with reference to FIG. 14.
FIG. 14 shows the arrangement of a conventional spread spectrum communication system.
As is obvious from a comparison between FIG. 14 and FIG. 1 showing the arrangement of the first embodiment of a spread spectrum communication system according to the present invention, the prior art shown in FIG. 14 differs from the first embodiment of the spread spectrum communication system according to the present invention only in that each base station does not include a reception timing difference calculating section 183.
A description of an example of the conventional spread spectrum communication system in FIG. 14 overlaps that of the first embodiment of the spread spectrum communication system according to the present invention in FIG. 1, and hence will be omitted.
As the first example of the prior art in which the field of the invention is similar to that in the present invention, xe2x80x9cSynchronization Establishment Method for Spread Code in Mobile Communication System, Mobile Station Equipment, and Base Station Equipmentxe2x80x9d disclosed in Japanese Patent Laid-Open No. 9-275582 is presented.
According to the first example of the prior art, to quickly and efficiently perform soft handover, phase information about a spread code in a communication channel transmitted from a soft handover destination base station is received and the received phase information is notified to a soft handover source base station before a start-up of soft handover.
As the second example of the prior art in which the field of the invention is similar to that of the present invention, xe2x80x9cMobile Communication Reception Method and Devicexe2x80x9d disclosed in Japanese Patent Laid-Open No. 10-93532 is presented.
According to the second example of the prior art, when a plurality of signals each having a sufficient energy to be allocated for demodulation are received, it can be determined whether the signals are transmitted from the sector transmitter of the same base station or from another base station.
In addition, as the third example of the prior art in which the field of the invention is similar to that in the present invention, xe2x80x9cInitial Synchronization Method in Asynchronous Cellular System between DS-CDMA Base Station and Receiverxe2x80x9d disclosed in Japanese Patent Laid-Open No. 10-126380 is presented.
According to the third example of the prior art, in an initial site search, a long code timing is detected by using a matched filter, and a long code is specified using the long code timing detected by a plurality of parallel-connected correlators, thereby realizing a high-speed initial site search.
As described above, various spread spectrum mobile communication systems have been proposed. In a conventional spread spectrum mobile communication system, as described above, in handover between sites (handover in this specification includes both soft handover and hard handover), the reception timing of a reception wave in a handover destination site differs from that in a handover source site.
For this reason, in the handover destination site, a reception timing must be obtained by making a new search in a wide range up to the maximum propagation delay in the site. In hard handover, no signal can be received from a mobile station during execution of path acquisition, resulting in a short break.
In the worst case, no path can be acquired, and handover may fail.
In addition, in soft handover, selective diversity cannot be used during execution of path acquisition.
Furthermore, in the conventional spread spectrum communication system, each mobile station requires a 1-frame search time for a reception timing, e.g., 10 ms, resulting in a long period of processing time.
In the conventional spread spectrum communication system, a mobile station makes a cell search in the following manner.
1. The power to the mobile station is turned on.
2. A perch channel exhibiting the highest reception level at that point is received with a common spread code, and this perch channel is regarded as a main perch channel.
3. Perch channels from neighboring sites are received to prepare for handover to a neighboring site.
4. When xe2x80x9cmain perch channel reception levelxe2x80x9d becomes lower than xe2x80x9cperch channel reception level in neighboring site exhibiting highest reception levelxe2x80x9d, handover is performed to the neighboring site exhibiting the highest perch channel reception level.
A cell search made by a mobile station in such a conventional spread spectrum communication system is described in detail in xe2x80x9cReception Method and Apparatus in CDMA Radio Communicationxe2x80x9d disclosed in Japanese Patent Laid-Open No. 10-94041 and xe2x80x9cInitial Synchronization Method and Receiver in DS-CDMA Base Station asynchronous Cellular Systemxe2x80x9d disclosed in Japanese Patent Laid-Open No. 10-126380 (described above).
As described above, in a cell search made by a mobile station in the conventional spread spectrum communication system, although the transmission timing of a perch channel in each sector of a base station in the same cell (base station) is known, the transmission timings of perch channels are asynchronous (vary) among cells (base stations). For this reason, a reception timing must be detected again from an unknown state for another cell (base station).
For this reason, it takes a long processing time to receive perch channels of other cells (base stations) (about six cells in general).
The present invention has been made in consideration of the above situation, and has as its object to provide a spread spectrum communication system which accumulates past reception timing differences in handover between sites, and corrects a reception timing in a handover destination site in handover between sites with the accumulated reception timing differences, thereby quickly demodulating signals in a handover destination site without making a path search in a wide range, and a handover method in the spread spectrum communication system.
It is another object of the present invention to provide a spread spectrum communication system which accumulates past reception timing differences in handover between sites, and corrects a reception timing in a mobile station in handover between sites with the accumulated reception timing differences, thereby quickly demodulating signals in the mobile station without making a path search in a wide range, and a handover method in the spread spectrum communication system.
In order to achieve the above objects, according to the present invention, there is provided a spread spectrum communication system comprising at least one mobile station and a plurality of base stations for communicating with mobile stations in sites thereof by a spread spectrum scheme, each of the base stations including accumulation means for accumulating a reception timing difference indicating a difference in reception timing between a transmission signal from a mobile station in a base station in an adjacent site and a transmission signal from the mobile station in a self-station, and calculation means for obtaining a reception timing of the transmission signal from the mobile station in the self-station by using a reception timing difference between the self-station and a handover source base station in an adjacent site, which is accumulated in the accumulation means, when the self-station become a handover destination base station upon handover of the mobile station between adjacent sites.